1. Field of the Invention
This invention relates to a memory cell configuration that is implemented so as to be relatively hard to the adverse effects of nuclear radiation.
2. Prior Art
Exposure of a conventional semiconductor memory cell to a nuclear radiation event may result in both permanent degradation of component parameters as well as a temporary upsetting of cell operation. Permanent degradation in the characteristics of the semiconductor components which comprise the memory cell is typically caused by neutron and total ionizing radiation dose effects. More particularly, a permanent degradation in component characteristics may result in a shifting (i.e. increase or decrease) in the threshold potentials of complementary semiconductor devices, as well as an increase in cell leakage current and a decrease in gain.
In addition, temporary upsets induced by the rapid anneal effects from a nuclear event may result in momentary shifts in the parameters of the semiconductor components which comprise the memory cell. Such rapid anneal effects are common when a memory cell is exposed to a large total ionizing radiation dose from x-rays and gamma rays. The rapid anneal effects may temporarily cause a transient shift in the threshold potentials of the components which comprise the memory cell. As the rapid anneal effects dissipate with time, the parameters of the components subsequently recover to the level of permanent degradation.
What is more, transient radiation is known to connect an equivalent impedance (i.e. a shunt conductance) across the components which comprise a memory cell during a nuclear radiation event. Such transient upsets act to undesirably alter the performance of the memory cell.
Hence, as a consequence of the nuclear event, the resulting ionizing radiation can cause the destruction of previously stored data in a memory cell or create externally applied unbalances which may undesirably switch the logic state of or inhibit the ability to read a selected memory cell from an array thereof. Moreover, the ionizing radiation from a nuclear event may undesirably scramble the associated interface logic which controls such an array of memory cells so that the selection of one memory cell may adversely interfere with the operation of other memory cells comprising the array.
One example of a prior art memory cell can be found in U.S. Pat. No. 3,990,056 issued Nov. 2, 1976.